The level of power dissipation in modem electronic devices is an ongoing concern for designers. It is important for the designer to extend battery life in portable electronic devices, such as digital cameras and cell phones, by controlling power consumption. It is also important to limit the heat dissipated within the integrated circuits used in such portable devices, as well as in other applications. Heat dissipation in an integrated circuit can be a major design and operational constraint. Increasingly, solid state imaging devices such as CMOS and other image sensor arrays used, for example, in digital camera and cell phone modules are being designed to capture images with greater and greater resolution. The capture of images of ever greater resolution has resulted in image sensor chips with increased numbers of pixels on the image sensor arrays and with associated increased pixel densities. The resulting pixel densities and related signal processing circuitry on such chips, as well as the faster clock speeds necessary to capture the higher resolution images within an acceptable time, has resulted in greater power densities dissipated in these image sensor chips.
Each pixel in an image sensor array converts the light hitting it in a fixed period of time into electronic charge which is transferred out of the array in a preselected order and detected as a voltage signal. This analog voltage signal from the array is then converted into a digital signal. Prior to the analog-to-digital (ADC) stage there is often an amplifier with programmable gain that increases lower level signals resultant from lower light level conditions in an attempt to utilize the full range of the ADC stage.
In addition, each pixel typically has a color filter over it so that each pixel is responsive to light only in a given frequency band typically corresponding to either the red, green, or blue colors. In addition to the color filter, the silicon of the pixels converts different frequencies of light with different efficiencies. The conversion efficiency is greater for the red color band than for the blue color band. Thus, the amplification needed for each of the color band signals will be different.
For economic reasons in fabricating integrated circuits it is important to keep the size of the integrated circuit die small. As such, increasing the number of pixels in an image sensor array results in an effort to reduce the size of the individual pixels, so as to keep the overall die size as small as possible. The smaller pixel sizes reduce the light sensitivity of the pixels and cause a need for higher programmable amplifier gains. Higher pixel counts lead to faster system clock speeds in order to be able to capture an image in a desired period of time. Chip heating due to the amplification stages increases as the gain and speed requirements of the amplifiers increase. Localized heating on the integrated circuit die in the region of the image array can cause image artifacts.
Typically when an amplifier is designed, the requirements for the amplifier in terms of gain and frequency response are determined. The amplifier is designed so that it can achieve the desired output settling at the maximum bandwidth (speed of signal capture and processing) while amplifying the signal at the maximum gain setting. If the amplifier can meet this maximum Gain-Bandwidth requirement, then the amplifier is able to handle all other combinations of lower gain and lower bandwidth.